1. Field of the Invention
The present invention relates to a rack for pipette cones, in particular for multichannel pipettes.
2. Background of the Invention
Racks presenting a plane support surface in which orifices are provided in a rectangular matrix are known, e.g. from document U.S. Pat. No. 3,494,210. A rack of that type can receive cones in the orifices, with each cone having, for example, a shoulder suitable for bearing on the support face. The rack enables a plurality of cones to be fixed to the row of endpieces of a multichannel pipette. Since the spacing between the endpieces corresponds to that of the cones received in the rack, the pipette is placed in such a manner that all of its endpieces penetrate into respective cones in a row of orifices in the rack. The user then presses the pipette against the rack so that the cones are retained on the endpieces by friction. Thereafter the user moves the pipette away, together with the cones retained on the endpieces. Nevertheless, that technique generally does not give satisfaction. When the user presses the pipette against the rack, the rack sags in the middle of the row of cone-receiving orifices. As a result, either the user does not press hard enough so when the pipette is moved away some endpieces are not carrying cones, in particular endpieces in the middle of the row, or else the user presses hard and as a result even if there is one cone on each endpiece, it will subsequently be necessary to pull hard in order to separate the cones from the endpieces, particularly for endpieces at the ends of the row.
In addition, the cones, the rack, and the endpieces naturally have geometrical defects, limited to some extent by manufacturing tolerances. Unfortunately, that gives rise to difficulties when the defects are cumulative in one direction for one endpiece and its associated cone, and in an other direction for another endpiece and its associated cone. This happens for a pipette that possesses a first endpiece whose outside width corresponds to the maximum tolerance and that receives a first cone whose inside diameter corresponds to the minimum tolerance, and a second endpiece of the opposite configuration, i.e. of outside width corresponding to a minimum tolerance receiving a second cone whose inside diameter corresponds to the maximum tolerance. Under such conditions, with the above-mentioned rack, and even when no significant sagging occurs, the first cone will be mounted tightly on the first endpiece well before the second cone is securely mounted on the second endpiece. Since the mutually-contacting faces of the cones and the endpieces are conical, that means that the friction zones between the cones and the endpieces on the first and second endpieces are at different heights. This can be made worse by geometrical faults relating to contact between cones and the support surface. Under such conditions, the greater the geometrical defects in the cones, the endpieces, and the rack, the more difficult it becomes to put cones effectively on all of the endpieces of a given pipette.
An object of the invention is to mitigate those drawbacks and to provide a rack that makes it possible to fix cones effectively on all of the endpieces of a pipette while requiring only moderate force from the user.
To achieve this object, the invention provides a rack for pipette cones, in particular for a multichannel pipette, the rack having aligned orifices forming at least one row in at least one alignment direction, and a support face adapted to support cones received in the orifices, the rack making the cones of the row accessible simultaneously, the support face having a transverse profile in the alignment direction that is generally convex in shape.
Thus, the convex shape prevents a concave sag appearing while the cones are being fixed on the endpieces. This ensures that cones are fixed properly on all of the endpieces of the pipette, and in particular on the endpieces in the middle of the row. This fixing can be obtained with the user exerting moderate force only. Thereafter, the force that needs to be supplied to separate the cones from the endpieces is likewise moderate. In addition, the rack of the invention avoids the need to manufacture the rack, the endpieces, and the cones to comply with very tight dimensional tolerances. Furthermore, it is not necessary to provide the rack with reinforcing ribs. The rack can thus be manufactured, in particular by molding, in a manner that is fast and of low cost. Also, the rack makes it possible to load each cone on the corresponding endpiece easily and without excessive tightening, without being troubled by any geometrical and dimensional defects of the cones, the endpieces, and the rack. To achieve this, it is sufficient, for example, to use the method of the invention for fixing cones to endpieces by tilting, in the manner described below.
The shape need not be xe2x80x9cconvexxe2x80x9d within the strict meaning of the word, for example it can be convex overall while being concave locally. For example, the profile could be M-shaped or camel-humped. Thus, the profile may present at least one upwardly-projecting zone. The profile can have at least one middle zone, preferably in its center, which is higher than the two ends of the profile. In some cases, the shape can be strictly convex.
Advantageously, the orifices are aligned in two non-mutual parallel alignment directions, the support face having transverse profiles in both alignment directions that are convex in shape.
Thus, depending on the number of endpieces on the pipette, the rack can be used with a row of orifices in one or other of the two alignment directions, while obtaining the same above-mentioned advantages.
Advantageously, the orifices are aligned so as to form at least two rows in two respective non-mutually parallel alignment directions, the rack. making the cones in each row accessible simultaneously.
Advantageously, the profile or at least one of the profiles, is rounded in shape.
Advantageously, the profile, or at least one of the profiles, is in the form of a circular arc.
This shape is particularly effective for providing a good distribution of forces on the endpieces for fixing each cone by friction.
Advantageously, the support face is spherical in shape.
Advantageously, the profile, or at least one of the profiles, is triangular in shape.
Advantageously, the rack is constituted by a plurality of portions that are substantially unmovable relative to one another.
Advantageously, the rack is made as a single piece.
Advantageously, the rack has a support wall defining the support face, and support means connected directly to the support wall and suitable for supporting it by resting on a base.
Advantageously, the general shape of the rack is that of a rectangular parallelepiped.
The invention also provides a method of fixing cones on the endpieces of a multichannel pipette in which, starting with a pipette having at least first and second endpieces and with a rack of the invention receiving at least first and second cones in its orifices, the method comprises the following steps:
placing the pipette relative to the rack in such a manner that the profile lies in the same plane as the axes of the endpieces, the first endpiece penetrating into the first cone so as to hold the first cone on the first endpiece, and the second endpiece extending in register with and at a distance from the second cone; and
tilting the pipette while keeping the profile in the same plane as the axes of the endpieces so that the first cone held on the first endpiece moves to a distance from the support face and so that the second endpiece penetrate into the second cone so as to hold the second cone on the second endpiece.
This method makes it possible to benefit to a very large extent from the advantages of the rack of the invention. Contrary to the known method in which the pipette is pressed against the rack so as to fix cones simultaneously on all of the endpieces, the method of the invention makes it possible to fix cones successively on the endpieces, endpiece-after-endpiece along the row of cones; The user can thus supply very moderate force which is nevertheless sufficient to put each cone individually into place while simultaneously ensuring that the cone is very well retained by friction on the endpiece.
The invention also provides a method of fixing cones on the endpieces of a multichannel pipette in which, starting from a pipette having at least two endpieces and a rack of the invention receiving at least two cones in its orifices, the method comprises the steps of:
placing the pipette relative to the rack in such a manner that the profile lies in the same plane as the axes of the endpieces, the endpieces penetrating into the respective cones; and
urging the pipette towards the rack parallel to the axes of the endpieces so as to retain the cones simultaneously on the respective endpieces.
Thus, in this case, the force provided by the user deforms the support face elastically by flattening its convex shape so as to fix all of the cones effectively to the endpieces.